My parents *only* speak Hokkien / Taiwanese to each other, but they speak to me in Mandarin / Hokkien / English.
It would be weird to switch languages. I have cousins I speak certain languages to but not others, even if we are all proficient in something else as well. It just feels.. weird.
I never figured out why or how this sort of language rigidity starts or how it’s even established or maintained.